This invention relates to a method for avoiding radar detection and to a radar absorbing material for use therewith. In a more particular aspect, this invention concerns itself with the application of a radar absorbing ferrite material to a radar reflecting body.
The recent advent of high altitude aircraft and missiles for military applications has generated considerable interest in the development of methods for preventing their tactical discovery by enemy radar and infrared detectors. Effective concealment can be achieved provided that the exposed surface of aircraft or missile is capable of (1) absorbing incoming radar waves and (2) have a low reflectance in the infrared spectrum. It has been found that these conditions can be met by coating the exposed surfaces with spinel ferrites having Curie temperatures in excess of 700° F. However, considerable difficulty has been encountered during attempts to satisfactorily apply the ferrites to surfaces. The coatings do not possess sufficient adhesion or the mechanical and chemical stability needed to withstand the thermal shock and vibration encountered during operation in a high altitude environment as well as the severe conditions of stress and strain associated with engine startup and shutdown.
In addition, many advanced military aircraft are equipped with missiles guided by both radar and infrared (IR) homing devices. For aft-hemisphere attack, primary return source for both microwave and infrared frequencies is the (hot) cavity of the engine exhaust. In the case of IR frequencies, radiation from hot internal engine parts can be reflected from tailpipe liner or plug surfaces (depending on engine configuration) even if these surfaces are cooled by turbine bypass air. Uncooled surfaces can of course radiate directly to increase the IR signature. For microwave frequencies, radiation transmitted from the attacking vehicle is returned in large measure from internal engine parts and flame holders through multiple reflections or propagation by wave guide modes in the engine exhaust cavity. Return of on-axis components is strongly dependent on engine exhaust design features. Components which are greater than 10°-15° off-axis generally are reflected many times inside the exhaust cavity before being re-radiated in the aft-hemisphere and are particularly susceptible to attenuation by a well designed absorber system.
In order to effectively reduce both the aft-hemisphere radar cross-section (RCS) and IR signature of an advanced military aircraft, it has been discovered that a combined high temperature radar absorbant-low infrared reflectance material in the form of a thin, non-reflecting coating of a spinel ferrite can be applied to those engine parts and structural elements that are involved in the reflection and scattering of radar signals. The coating composition can be applied to metal substrates by any well known coating technique, and is composed of a powdered mixture of a spinel ferrite and a glass frit suspended in water. In this way the desired ferrite material can be effectively bonded to metal substrate in order to prevent or minimize the detection of military aircraft and missiles by their radar or infrared signatures.